Assembly method for a housing, assembly system and associated housing

Abstract

Assembly method for a housing, assembly system and associated housing. The present invention relates to a method for assembling a housing comprising an electronic board, a solder bead and a cover, the method comprising at least the following steps: positioning the electronic board inside a closed enclosure; positioning a solder preform and the cover, the solder preform being interposed between the electronic board and the cover in a stacking direction; injection of a sealing gas until a first sealing gas pressure (P1) is reached; raising the temperature to heat the solder preform, the electronic board, the solder preform and the cover being free to move; when the solder preform has reached a first temperature threshold (T1), further injection of sealing gas until a second pressure (P2) is reached; and lowering the temperature inside the enclosure.Figure for the abbreviation: Figure 4.

Description

Title of the invention: Method for assembling a housing, assembly system and associated housing

[0001] The present invention relates to a method for assembling a housing, an assembly system and a housing associated with the method.

[0002] To assemble housings containing, for example, precision sensors, it is known to solder together an electronic board containing the sensors and a cover. This is achieved using a solder preform, positioned between the electronic board and the cover. Once assembled, the housing thus formed must be airtight. For this to happen, it is necessary that the electronic board, the solder preform, and the cover be aligned and remain aligned throughout the entire assembly process.

[0003] It is known to assemble the housing under vacuum, possibly by mechanically holding the parts to be assembled. However, vacuum brazing entails additional costs, as well as risks of cracks in the brazing bead formed after assembly, or in the housing.

[0004] It is also known to assemble the housing under a controlled atmosphere, while mechanically holding the parts together during assembly, which increases the complexity of the assembly process, as well as its cost. It is also known to close the assembled housing with a plug and then fill the housing with gas during passage through a so-called "glove box" enclosure. However, this increases the number of steps required to assemble the housing and the number of parts, since a plug and a sealing gasket must be manufactured and assembled. The assembly process is therefore lengthy and complex, and the additional parts generate additional costs.

[0005] The aim of the invention is therefore to offer a solution for assembling a case quickly and simply.

[0006] To this end, the invention relates to a method for assembling a housing comprising an electronic board, a solder cord and a cover, the method comprising at least the following steps:

[0007] - positioning of the electronic board inside a closed enclosure, the board electronics including a cavity;

[0008] - positioning of a brazing preform and the cover inside the enclosure closed, the solder preform framing the cavity and being interposed between the electronic board and the cover in a stacking direction;

[0009] - vacuum sealing of the inside of the closed enclosure;

[0010] - injection of a sealing gas inside the closed enclosure until reaching an initial pressure of the sealing gas;

[0011] - increasing the temperature inside the closed enclosure to heat the brazing preform,

[0012] the electronic board, the soldering preform and the cover being free to move;

[0013] - when the brazing preform has reached a first temperature threshold, the first where the temperature threshold is greater than or equal to the melting temperature of the brazing preform, additional sealing gas is injected into the closed chamber until a second sealing gas pressure is reached, strictly greater than the first pressure; and

[0014] - decrease in temperature inside the enclosure until the preform the solder reaches a temperature strictly lower than the melting temperature of the solder preform, the solder preform then forming a solder bead mechanically connecting the electronic board and the cover,

[0015] the solder cord, the electronic board and the cover delimiting an interior space of the housing.

[0016] Thanks to the invention, it is not necessary to mechanically hold the parts during assembly. Indeed, the suppression of the gas inside the enclosure keeps the cover in place, thus preventing defects such as misalignment of the cover and the electronic board, or the appearance of bubbles or leaks in the solder bead. These defects are due, for example, to the expansion of the gas inside the housing after the solder preform has already melted, which generates pressure on the cover that can displace it, or generate bubbles or leaks in the solder bead if there is no overpressure of the gas in the closed enclosure.

[0017] Thanks to the invention, the assembly process does not require mechanical means for holding the parts, the addition of extra parts, or heating the brazing preform under vacuum. The assembly is thus simple, inexpensive, and reduces the risk of cracking.

[0018] According to other advantageous aspects of the invention, the method comprises one or more of the following features, taken individually or in all technically possible combinations:

[0019] - when the brazing preform and the hood are positioned, the cavity dries, the drying stage comprising at least one injection of inert gas into the closed chamber followed by a vacuuming of the interior of the closed chamber; and

[0020] - injection of an oxidoreducing gas inside the closed enclosure;

[0021] - when a sealing gas pressure is substantially equal to the second pressure, temperature increase inside the closed chamber so that the brazing preform reaches a second temperature threshold,

[0022] and in which the decrease in temperature inside the closed enclosure until the temperature of the brazing preform becomes strictly less than the melting temperature of the brazing preform is carried out when a predetermined time, measured from a moment when the brazing preform has reached the second temperature threshold, has elapsed;

[0023] - the second temperature threshold is within a range of 20°C to 50°C at- above the melting temperature of the brazing preform;

[0024] - the first pressure is between 0.8 and 1.2 bar, preferably substantially equal to 1 bar and the second pressure is between 1 and 1.5 bar, preferably approximately equal to 1.3 bar;

[0025] - the sealing gas comprises nitrogen and a tracer gas, the tracer gas being helium;

[0026] - the brazing preform is formed from a eutectic alloy;

[0027] - the electronic board includes a microelectromechanical system, located in the cavity, and following the positioning step of the brazing preform and the hood, the hood covers the microsystem.

[0028] The invention also relates to an assembly system comprising a closed enclosure, means for vacuuming the closed enclosure, means for injecting gas into the closed enclosure, means for heating the closed enclosure and electronic control means, the system being configured to implement the process described above.

[0029] The invention also relates to a housing obtained by the process described above.

[0030] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which: - [Fig.1] [Fig.1] is a diagram of an electronic board, a solder preform and a cover according to the invention; - [Fig.2] [Fig.2] is a cross-sectional view of a housing according to the invention; - [Fig.3] [Fig.3] is a diagram of an assembly system according to the invention; - [Fig. 4] [Fig. 4] is a graph showing the evolution of the temperature of the brazing preform and the pressure inside an enclosure over time; and - [Fig.5] [Fig.5] is a flowchart of an assembly process according to the invention.

[0031] Figure 1 shows an electronic board 10, a cover 12, and a solder preform 14. The electronic board 10 includes a cavity 16. Advantageously, the The electronic board 10 comprises a substrate 17, which extends substantially parallel to a plane P. Advantageously, the cavity 16 is formed in the substrate 17; in particular, one of the faces of the substrate 17 parallel to the plane P includes the cavity 16. The substrate 17 is advantageously made of ceramic and coated with a film, for example, a silver film (not shown). Alternatively, the silver film is deposited only on one face of the substrate 17, in particular the face of the substrate 17 containing the cavity 16.

[0032] Advantageously, and as shown in Figures 1 and 2, the electronic board 10 includes a sensor 18, located in the cavity 16. The sensor 18 is, for example, a microelectromechanical system, or MEMS. Alternatively, an electronic system, comprising, for example, a plurality of electronic chips, is located in the cavity 16.

[0033] The hood 12 is advantageously made of a metal alloy, for example a nickel-gold alloy. The hood 12 comprises at least one, here four, side walls 20, which support a main face 22 of the hood 12. Advantageously, the hood 12 is made in one piece.

[0034] The solder preform 14 forms a closed contour, the shape of which follows the shape of the side walls of the cover 12, projected onto plane P. In the example of [Fig. 1], the side walls of the cover 12 form a rectangle projected onto plane P. Thus, the solder preform 14 is rectangular in shape. The solder preform 14 is advantageously made of a eutectic metal alloy. A eutectic alloy is understood to be a homogeneous mixture of two or more pure substances that melts at a constant temperature as a single substance. For example, the solder preform 14 is made of a eutectic tin-silver alloy, or, alternatively, a gold-tin alloy. The material of the solder preform 14 is advantageously chosen to ensure good wettability with the electronic board 10 on the one hand, and with the cover 12 on the other.

[0035] For example, in the case where the substrate 17 is coated with silver and the hood 12 is made of nickel-gold alloy, the brazing preform 14 is advantageously made of tin-silver alloy.

[0036] A housing 30, visible in [Fig. 2], is formed by assembling the electronic board 10, the cover 12 and the solder preform 14. When the housing 30 is assembled, the cover 12 is arranged on the electronic board in a stacking direction X perpendicular to the plane P. Advantageously, the cover 12 is arranged on the substrate 17. Advantageously, the cover 12 covers the cavity 16 and the sensor 18.

[0037] When the housing 30 is assembled, the solder preform forms a solder bead 14', which mechanically connects the electronic board 10 and the cover 12. Advantageously, the solder bead 14' frames the cavity 16. The electronic board 10, the cover 12, and the solder bead 14' then form a mechanically bonded assembly. The housing 30 is airtight, meaning that a gas present inside the housing 30 cannot escape to the outside of the housing 30, and conversely, a gas outside the housing 30 cannot infiltrate the inside of the housing 30. Thus, in the example of Figures 1 and 2, the sensor 18 is isolated from the outside. This ensures reliable performance of the sensor 18 over time, without interference caused by humidity or external air pollution.

[0038] In order to assemble the electronic board 10, the cover 12 and the solder preform 14 to form the housing 30, it is possible to use an assembly system 40, shown in [Fig.3].

[0039] The assembly system 40 comprises a chamber 42, the chamber 42 being a closed chamber, means for vacuuming the chamber 42, means for injecting gas 46 into the chamber 42, means for heating the chamber 42 and electronic control means 50, advantageously connected to the chamber 42, to the vacuuming means 44, the gas injection means 46 and the heating means 48. Advantageously, the assembly system 40 also comprises a pressure sensor 52, located inside the chamber 42.

[0040] The enclosure 42 is advantageously configured to be hermetically sealed.

[0041] The enclosure 42 and the heating means 48 together form, for example, an oven. The control means 50 are advantageously configured to control the assembly system 40. The control means 50 include, for example, a computer, comprising a human-machine interface through which a user controls the various elements of the assembly system 40. The control means 50 advantageously include at least one programmable logic component, such as an FPGA (Field Programmable Gate Array), or an integrated circuit, such as an ASIC (Application-Specific Integrated Circuit).

[0042] A method for assembling the housing 30 is described opposite Figures 4 and 5. The method includes at least the steps described below.

[0043] During a positioning step 102, the electronic card 10 is placed inside the enclosure 42. The positioning step 102 is carried out manually, or, advantageously, using an automatic gripping and placing machine, for example, an articulated arm.

[0044] During a positioning step 104, the solder preform 14 and the hood 12 are positioned inside the closed enclosure 42. In particular, the solder preform 14 is positioned so as to frame the cavity 16 and is interposed between the electronic board 10 and the hood 12 according to the stacking direction X.

[0045] Advantageously, the brazing preform 14 is deposited on the substrate 17, and the hood 12 is deposited on the brazing preform 14, so that the edge of the side walls 20 is in contact with the brazing preform 14 in the stacking direction X.

[0046] Positioning step 104 is advantageously carried out by the same automatic gripping and placing machine as that used during positioning step 102. The electronic board 10 and the cover 12 are not connected together in a sealed manner, gaps being present between the solder preform 14 and the electronic board 10 on the one hand and the cover 12 on the other hand.

[0047] Advantageously, once the positioning step 104 has been carried out, a drying step 106 is performed. The drying step 106 comprises at least one injection of inert gas, also called neutral gas, into the enclosure 42, for example via the gas injection means 46, followed by a vacuuming of the interior of the closed enclosure 42, for example via the vacuuming means 44. The drying step 106 thus allows the removal of any trace of water on the electronic board 10, including in the cavity 16, on the sensor 18, the cover 12 and the solder preform 14, as well as any residual moisture that may be present inside the enclosure 42.

[0048] Advantageously, the drying step 106 includes several cycles of inert gas injection followed by a vacuuming of the inside of the enclosure 42. The inert gas is chosen so that it does not react on contact with the parts 10, 12 and 14 inside the closed enclosure 42, and is for example dinitrogen, more simply called nitrogen in the following.

[0049] Advantageously, following the drying step 106, an injection step 108 is carried out. The injection step 108 is also called the deoxidation step 108. The deoxidation step 108 consists of injecting a redox gas into the enclosure 42. The deoxidation step 108 is implemented, for example, by gas injection means 46. The redox gas is, for example, formic acid in the gaseous phase. The deoxidation step 108 makes it possible to remove any oxide layer present on the surface of the electronic board 10, the solder preform 14, and the cover 12.

[0050] Advantageously, once the deoxidation step 108 has been carried out, a vacuum step 110 is performed. The vacuum step 110 is advantageously implemented by the vacuum means 44, and allows the gases present inside the enclosure 42 to be evacuated. For example, the vacuum step 110 allows the oxidizing gas and the oxidation residues formed in the deoxidation step 108 to be evacuated.

[0051] Once the vacuum-sealing step 110 has been completed, a sealing gas injection step 112 is performed inside the enclosure 42. The sealing gas injection is carried out until a first pressure PI of the sealing gas is reached. In other words, sealing gas is injected into the chamber 42 as long as a pressure P inside the chamber 42 is less than or equal to the first pressure PI. The first pressure PI is, for example, between 0.8 and 1.2 bar, preferably approximately equal to 1 bar. By approximately equal to a quantity, we mean equal to that quantity plus or minus 1%. The sealing gas injection step 112 is advantageously implemented by the gas injection means 46, and the pressure P inside the chamber 42 is advantageously measured by the pressure sensor 52. As an example, the sealing gas comprises an inert gas, such as nitrogen, and a tracer gas, for example, helium.

[0052] When the pressure P inside the enclosure 42 is substantially equal to the initial pressure PI, a step 114 of increasing the temperature inside the closed enclosure 42, also called the heating step 114, is carried out in order to heat the brazing preform 14, whose temperature T increases. The heating step 114 is advantageously carried out by the heating means 48.

[0053] Advantageously, the temperature T of the solder preform 14 is not measured directly, but is determined indirectly, for example by measuring the temperature of the substrate 17. In this case, a calibration is carried out prior to assembly to correlate the temperature of the electronic board 10, in particular of the substrate 17, to the temperature T of the solder preform 14.

[0054] During steps 106 to 114, the electronic board 10, the cover 12, and the solder preform 14 are not mechanically held. In other words, the electronic board 10, the cover 12, and the solder preform 14 are free to move. In practice, during steps 106 to 114, the gas injections and vacuuming of the interior of the enclosure 42 are sufficiently gradual so as not to cause any displacement of the parts 10, 12, or 14.

[0055] When the brazing preform 14 has reached a first temperature threshold Tl, an additional injection step 116 of sealing gas into the enclosure 42 is carried out, until a second pressure P2 of the sealing gas is reached inside the closed enclosure 42, as shown at time A in [Fig.4].

[0056] The first temperature threshold Tl is greater than or equal to a melting temperature Tfus of the brazing preform 14. In the example of [Fig.4], the first temperature threshold Tl is chosen to be greater than the melting temperature Tfus of the brazing preform 14.

[0057] Thus, when the brazing preform 14 has a temperature equal to the first temperature threshold Tl, the brazing preform 14 is entirely in a liquid state. It then forms a gas-tight barrier. The sealing gas inside a space 32 Delimited by the electronic board 10, the solder preform 14 in its liquid state and the cover 12 are thus confined within it. The space 32 is referred to as the internal space of the housing 30.

[0058] Advantageously, the additional injection step 116 is carried out by the gas injection means 46. Advantageously, the second pressure P2 is between 1 and 1.5 bar and is preferably substantially equal to 1.3 bar.

[0059] Advantageously, when the gas pressure P inside the enclosure 42 is substantially equal to the second pressure P2, a temperature increase step 118 inside the enclosure 42 occurs, so that the solder preform 14 reaches a second temperature threshold T2, as seen at time B in [Fig. 4]. The second temperature threshold T2 is advantageously strictly higher than the first temperature threshold TL. The second temperature threshold T2 is, for example, within a range of 20°C to 50°C above the melting temperature Tfus of the solder preform 14. The second temperature threshold T2 is advantageously chosen to optimize the diffusion of the atoms forming the alloy of the solder preform 14 into the electronic board 10 and the cover 12, and thus improve the strength of the solder joint.

[0060] Increasing the pressure P to the second pressure P2 advantageously allows the cover 12 to be pressed against the electronic board 10, preventing unwanted movement of the cover 12 and the formation of bubbles in the solder preform 14, which is now liquid. Unwanted movement of the cover 12 is caused, for example, by the expansion of the sealing gas in the internal space 32, this expansion being caused by the temperature increase required to heat the solder preform 14 from the first temperature threshold T1 to the second temperature threshold T2. Gas expansion can also occur without the need to heat the solder preform 14 to the second temperature threshold T2, and can be caused by the fact that the sealing gas in the internal space 32 reaches temperature T1 only after the solder preform 14 has reached this temperature and become liquid.

[0061] Advantageously, when the solder preform 14 reaches the second temperature threshold T2, its temperature T is maintained constant and substantially equal to the second temperature threshold T2 for a predetermined time D, measured from the moment when the solder preform 14 reached the second temperature threshold T2. When the time D has elapsed, a temperature decrease step 120 is carried out, as seen at time C in [Fig.4].

[0062] During the temperature reduction step 120, the temperature inside the enclosure 42 decreases. Thus, the temperature T of the solder preform 14, which is in a liquid state, also decreases. The temperature inside the enclosure 42 decreases until the temperature T of the solder preform 14 becomes strictly lower than the melting temperature Tfus of the solder preform 14. The solder preform 14, which was in a liquid state, solidifies and forms a solder bead 14', mechanically connecting the electronic board 10 and the cover 12. The housing 30 is thus formed.

[0063] The temperature reduction step 120 is implemented for example by the heating means 48, which stop heating the inside of the enclosure 42.

[0064] Advantageously, the temperature continues to decrease until it reaches a temperature of around 20°C, which allows the case 30 to be handled without risk of burning.

[0065] Thus, the assembly process described above makes it possible to obtain the housing 30. The solder bead 14', the electronic board 10 and the cover 12 delimit the internal space 32 of the housing 30, which is hermetically insulated from the outside.

[0066] In an alternative not shown, the temperature increase step 118 is not carried out, and the temperature decrease step 120 is carried out directly after the additional sealing gas injection step 116, advantageously after a predetermined time, measured from the moment when the pressure P in the enclosure 42 is equal to the second pressure threshold P2 is reached, has elapsed.

[0067] Any feature described for an embodiment or variant in the foregoing may be implemented for the other embodiments and variants described above, provided that it is technically feasible.

Claims

1. Demands Method for assembling a housing (30) comprising an electronic board (10), a solder bead (14') and a cover (12), the method comprising at least the following steps: - positioning (102) of the electronic card (10) inside a closed enclosure (42), the electronic card (10) comprising a cavity (16); - positioning (104) of a solder preform (14) and of the hood (12) inside the closed enclosure (42), the solder preform (14) framing the cavity (16) and being interposed between the electronic board (10) and the hood (22) according to a stacking direction (X); - vacuuming (110) of the inside of the closed enclosure (42); - injection of a sealing gas (112) inside the closed enclosure (42) until a first pressure (PI) of the sealing gas is reached; - increasing (114) the temperature inside the closed enclosure (42) to heat the brazing preform (14), the electronic board (10), the soldering preform (14) and the cover (12) being free to move; - when the brazing preform (14) has reached a first temperature threshold (Tl), the first temperature threshold (Tl) being greater than or equal to a melting temperature (Tfus) of the brazing preform (14), additional injection (116) of sealing gas into the closed enclosure (42), until a second sealing gas pressure (P2) is reached, strictly greater than the first pressure (PI); and - decrease (120) of the temperature inside the enclosure until the brazing preform (14) reaches a temperature (T) strictly lower than the melting temperature (Tfus) of the brazing preform (14), the brazing preform (14) then forming a bead of brazing (14') mechanically connecting the electronic board (10) and the cover (12), the brazing bead (14'), the electronic board (10) and the cover (12) delimiting an internal space (32) of the housing (30).

2. A method according to claim 1, further comprising the following steps: - when the brazing preform (14) and the hood (12) are positioned, drying (106) of the cavity (16), the drying step (106) comprising at least one injection of inert gas into the closed enclosure (42) followed by a vacuuming of the inside of the closed enclosure (42); and - injection of an oxidoreducing gas (108) into the inside of the closed enclosure (42).

3. A method according to any one of the preceding claims, further comprising the following step: - when a pressure (P) of the sealing gas is substantially equal to the second pressure (P2), increasing the temperature (118) inside the closed enclosure (42) so that the brazing preform (14) reaches a second temperature threshold (T2), and wherein the temperature inside the closed enclosure (42) is decreased until the temperature (T) of the brazing preform (14) becomes strictly less than the melting temperature (Tfus) of the brazing preform is carried out when a predetermined time (D), measured from a moment when the brazing preform (14) has reached the second temperature threshold (T2), has elapsed.

4. A method according to claim 3, wherein the second temperature threshold (T2) is within a range of 20°C to 50°C above the melting temperature (Tfus) of the brazing preform (14).

5. A method according to any one of the preceding claims, wherein the first pressure (PI) is between 0.8 and 1.2 bar, preferably substantially equal to 1 bar and the second pressure (P2) is between 1 and 1.5 bar, preferably substantially equal to 1.3 bar.

6. A method according to any one of the preceding claims, wherein the sealing gas comprises nitrogen and a tracing gas, the tracing gas being helium.

7. A method according to any one of the preceding claims, wherein the brazing preform (14) is formed of a eutectic alloy.

8. A method according to any one of the preceding claims, wherein the electronic board (10) comprises a microelectromechanical system (18), located in the cavity (16), and following the positioning step (104) of the solder preform (14) and the hood (12), the hood (12) covers the microsystem (18).

9. Assembly system (40) comprising a closed enclosure (42), means for vacuuming the closed enclosure (42), means for injecting gas (46) into the closed enclosure (42), means for heating the closed enclosure (42) and electronic control means (50), the system being configured to implement the method according to any one of claims 1 to 8.

10. Housing (30) obtained by the process of any one of claims 1 to 8.

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